Engines typically have a fuel system to store, pressurize, filter, and deliver fuel to the engine. The fuel system may use a pump to deliver fuel from the tank through a filter to the fuel control unit. The fuel control unit then feeds the fuel into the fuel pressure regulator which maintains the liquid fuel pressure being supplied to the engine. In order to run efficiently, the fuel received by the engine should quickly reach a specific pressure when the engine is started and maintain that pressure during operation.
However, pressure regulation issues may occur when a hot engine is turned off. For example, after an engine is turned off, the temperature of the engine and related components may continue to rise for a period of time as the engine undergoes a period of “heat soak.” A heat soak, or a hot soak, can cause fuel to boil inside the fuel lines and fuel filter. This may cause expansion of the vapor and any air in the vapor space, along with an increase in partial pressure of the fuel vapor. The pressure from the vaporized fuel, in turn, may push any liquid fuel remaining in the fuel lines back into the fuel tank. This can result in degraded start quality and increased emissions during a subsequent start.
In order to address this issue, some fuel systems incorporate a check valve between the pump and the fuel tank to reduce the amount of fuel that is pushed back to the fuel tank. However, this check valve still opens at a prescribed pressure and thus may still result in liquid fuel being pushed back into the fuel tank if the fuel pressure rises high enough. In other words, even with a check valve, vaporization of the fuel remaining in the fuel line may still occur, and fuel between the check valve and the fuel tank may run back to the fuel tank. As such, there still may be a potential for degraded start quality and increased emissions during a subsequent start.
Furthermore, the inventors herein have recognized that when using such a check valve, releasing the vapor pressure may also decrease the fuel pressure in the fuel delivery system once the system cools. Thus, not only does such a system allow for fuel to be pushed into the tank if temperature rises high enough, it also results in decreased pressure in the fuel line after the system cools. As a result, the operating pressure of the fuel system upon restart may take longer to reach a desired pressure or may be lower than expected, and this low pressure may result in degraded combustion, thereby increasing emissions and possibly contributing to poor start quality.